Reticle with antistatic coating

ABSTRACT

A static resistant reticle comprises a substrate and a patterning layer and is covered by an antistatic conductive film of quaternary amine (R 4 N) + Cl − . A pellicle structure comprising an optically transparent membrane tightly stretched on a frame is also coated by an antistatic electro conductive film of a similar material. The reticle with the pellicle form a shielded structure isolating the reticle from ESD.

RELATED APPLICATIONS

This is a continuation-in-part (CIP) application of U.S. patentapplication Ser. No. 10/273,682, filed Oct. 18, 2002.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to semiconductor industry, moreparticularly to reticles used in microlithography, and still moreparticularly to reticles with a thin film coating to isolate the maskfrom electric static field.

2. Description of the Related Art

A microlithography (or photolithography) process for manufacturing asemiconductor device comprises a photoresist application process, anexposure process, and a development process. During the photolithographyprocess, semiconductor manufacturers often use a photomask to copy animage of an electronic circuit onto a semiconductor wafer. Photomaskscome in various sizes and shapes such as 1X and 2X photomasks or 2X,2.5X, 4X, and 5X reticles. Depending on the reduction factor X, linewidth and line space geometries for a resulting semiconductor device arefrom less than 10 microns to less than 2 microns. Other mask linespacings and semiconductor line spacings also can be achieved. Whenworking with such small geometries, it is important that the reticle andother components in the fabrication processes be free of foreignparticles. A tiny speck of dust alters the desired pattern to be imagedonto the wafer. Conventionally, a thin transparent membrane, referred toas a pellicle membrane, is applied over the photomask portion of thereticle to keep the photomask portion free of foreign particles. Thepellicle membrane typically is positioned at a height above thephotomask. Such height is greater than the focal length of the lightimaged onto the photomask. Thus, small particles on the pelliclemembrane will not block light from reaching the photomask.

Reticles are a type of photomask that can be shot several times onto asingle wafer with a photolithographic tool known as a stepper orscanner. Photomasks generally include a fused silica (amorphous quartz)blank having a thin patterned opaque metal layer (e.g., chromium, orchrome) deposited on one surface of the blank. Typically, the layer isformed with chrome less than 100 nm thick and covered with ananti-reflective coating, such as chrome oxide. The purpose of theanti-reflective coating is to suppress ghost images from the lightreflected by the opaque material. This patterned metal layer containsthe microscopic image of the electronic circuit, which is frequentlyreferred to as the photomask's geometry. The quality of this geometrywill substantially dictate the quality of the electronic circuits formedon the semiconductor wafers from the photomask. As design rules havemoved toward smaller and denser integrated circuit (IC) devices, theintegrity of the photomask geometry has become increasingly important.

Due to specific conditions accompanying semiconductor-manufacturingprocess, namely low humidity (typically 40%±10% RH), it is especiallyprone to developing static electrical charges. Accumulated on thechrome, electrostatic charge leads to an uncontrolled electrostaticdischarge (ESD), one of the key causes of geometry degradation. ESD iscreated when a force causes a charge imbalance among a photomask'schrome structures. In the photomask context, effects of ESD includematerial sputtering and material migration. Specifically, depositing itsenergy into a very small area of the wafer, ESD irreversibly damages thelatter literally vaporizing metal conductor lines. Instances of theseeffects can result in the non-functioning of IC devices created from thedegraded photomask. It is not surprising, therefore, that prior artpresents a number of technical solutions addressing the ESD problem.

U.S. Pat. No. 5,989,754 issued to Chen et al. provides for a photomaskarrangement to prevent reticle patterns of a photomask from peelingcaused by electrostatic discharge damage. The photomask includes: asubstrate; a plurality of metal shielding layers formed on the surfaceof the substrate to provide the reticle patterns, wherein each two ofthe metal shielding layers are spaced apart by a clear scribe line; anda plurality of chrome lines formed on the clear scribe line to connectthe adjacent metal shielding layers, thereby increasing the effectivesurface area of the reticle patterns. Accordingly, when subjected to anaccidental electrostatic discharge, the chrome lines act as paths forspreading the static charge from one of the chrome lines to the other.

In U.S. Pat. No. 6,309,781 issued to Gemmink et al., a photomaskcomprises a transmissive base plate, a first side of which is providedwith a layer of a metallic mask material. In this layer, a mask patternis formed, which is enclosed in an outer region of mask material. Thephotomask is encapsulated in a protection layer of transmissive andconductive material, which, on the first side of the base plate, isformed at such a distance from the first side that the protection layerremains free of the mask pattern. The photomask is thus protectedagainst electrostatic discharges, which could damage the mask pattern,and the projection of the mask pattern is not adversely affected.

U.S. Pat. No. 6,359,313 issued to Yang et al. teaches an ESD protectiontransistor for discharging current from an ESD event present on aninput/output pad. The ESD protection transistor is capable of improveddischarging of excessive current without damage to the semiconductordevice and to the ESD protection transistor itself. The ESD protectiontransistor includes a first conductive line connecting an input/outputpad to the source and drain of the transistor at multiple pointspreventing the convergence of an excessive current at a certain pointand ESD damage to the transistor. The transistor also includes a secondconductive line formed on an insulating layer such that it does notoverlap with the first conductive line.

SUMMARY OF THE INVENTION

A primary object of the present invention is to provide an antistaticprotection for a reticle that would be compatible with existingreticle-pellicle structure and where potential damage from ESD would beeliminated.

According to the present invention, a reticle comprises a quartzsubstrate and a chrome pattern placed on the substrate and is providedwith a film coating of a conductive material that exhibits a shieldingeffect protecting the reticle by eliminating potential difference on thereticle.

An antistatic material used for coating includes quaternary amine(R₄N)⁺Cl⁻ with M.W. of about 300-500.

The amine used for the coating has chemical formula:

wherein n is an integer larger than 1.

The amine is used in the form of a 0.2˜5% solution (w/w) in isopropylalcohol or deionized water.

The solution is deposited by a spin coating process.

Alternatively, vapor coating may be used for depositing antistaticmaterial on the reticle substrate with subsequent drying.

The reticle with a film deposited on it is subjected to drying in aMarangoni-type dryer in the presence of nitrogen for the duration of4.5-6 min.

The Marangoni-type drying is followed by a warm air-drying at about 82°C. in the presence of nitrogen for about 8 min.

The film has thickness of about 500-1000.

A pellicle for a reticle comprises an I-line cellulose substrate that iscoated by an antistatic coating to prevent ESD on the reticle fromoccurring.

An antistatic material used for coating includes quaternary amine(R₄N)⁺Cl⁻ with M.W. of about 300-500.

The amine used for the coating has chemical formula:

The pellicle film is deposited according to the invention by animmersion of the substrate into a 0.2-1.5% (w/w) water or low-endalcohol solution of the amine.

The pellicle film is deposited by spin coating on the substrate.

The substrate is made of nitrocellulose of (C₆H₉O₇N)_(n) composition.

The coating deposited on the film preferably has thickness of about 100Å.

The pellicle is provided by a frame made preferably of aluminum.

The pellicle frame is also made coated with an antistatic dissipativefilm.

An antistatic material used for coating includes quaternary amine(R₄N)⁺Cl⁻ with M.W. of about 200-500.

The amine used for the coating has chemical formula:

The film can be deposited on the frame by immerse coating of the framewith subsequent natural drying.

The film deposited on the frame preferably has thickness of about 1.5μm.

BRIEF DESCRIPTION OF DRAWINGS

The above-mentioned and other objects, advantages and features of thepresent invention will be in more detail illustrated in and will becomemore apparent to those skilled in the art from the ensuing specificationand subjoined claims, when considered in conjunction with theaccompanying drawings, in which:

FIG. 1 schematically shows a mask (with a pellicle) according to thepresent invention;

FIG. 2 a is a schematical cross section of a front view of thepellicle-reticle structure;

FIG. 2 b is a plan view of FIG. 2 a;

FIG. 3 a is an illustration coating deposit in the beginning of theprocess;

FIG. 3 b is a cross-section of a pellicle structure according to thepresent invention;

FIG. 4 schematically shows depositing a film coating on a pellicle frameaccording to the present invention;

FIG. 5 schematically illustrates neutralizing an electrical charge,which otherwise would accumulate on the mask, according to the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, a photomask (reticle) 10 has a quartz blank 12with a thin patterned opaque metal layer 14, preferably made of chromium(chrome), deposited on a surface 16 of the blank 12. Typically, thelayer 14 is formed with chrome less than 100 nm thick and covered withan anti-reflective coating, such as chrome oxide (not shown). A pelliclemembrane 18 is arranged over the layer 14 of the reticle to keep it freeof foreign particles. A frame 20, over which the membrane 18 is tightlystretched, allows positioning the latter at some height above thephotomask. Such height (usually, of about 5-10 mm) is greater than thefocal length of the light imaged onto the photomask. Thus, a dustparticle that found its way on the pellicle is out of focal plane and isinvisible to the projection optics. A conventional pellicle frame ismade of anodized aluminum.

According to the present invention, a layer 22 of an antistatic film isdeposited onto the reticle 10. The layer 22 may be deposited by anyconvenient method known in the art. Examples of such methods are spincoating process where an agent is dropped on the surface of thesubstrate and is dispersed on the substrate surface using a centrifugalforce generated by rotating the substrate, or preferably, a chemicalvapor deposition process. Spin coating and CVD processes are disclosedin numerous sources, of which “Semiconductor Manufacturing Technology”by M. Quirk and J. Serda, Prentice Hall, 2001, pp. 264-282, 343,348-360, can be mentioned and is hereby incorporated by reference forall purposes.

An antistatic agent used for coating is a cationic polymer of quaternaryamine (R₄N)⁺Cl⁻ with a molecular weight of about 300˜500, whose chemicalformula is:

The inventors found that the CVD followed by Marangoni drying is thecombination that results in coating of better uniformity. Well known tothose skilled in the art, Marangoni drying is an ultra-clean dryingprocess, which relies on surface-tension gradient forces, so-calledMarangoni stresses. This method was found to be of particular use in thesemiconductor industry wherein obtaining ultra-clean surfaces is ofparamount importance. A surface tension gradient driven flow (aMarangoni flow) is used to remove the thin film of water remaining onthe surface of the reticle following rinsing. The process passivelyintroduces by natural evaporation and diffusion of minute amounts ofalcohol such as isopropyl alcohol (IPA) (or other suitable material)vapor in the immediate vicinity of a continuously refreshed meniscus ofdeionized water (DIW) or another aqueous-based, non-surfactant rinsingagent. An example of the Marangoni-type drying process is described inJ. Marra, “Ultraclean Marangoni Drying,” Particles in Gases and Liquids3: Detection, Characterization, and Control, Edited by K. L. Mittal,Plenum Press, New York, 1993, which is hereby incorporated by referencefor all purposes.

For depositing the coating, a 0.2˜5% w/w (weight in weight) solution ofthe agent in IPA is prepared. The reticle is subjected to two cycles ofdump rinse with the use of DIW. Then Marangoni drying with the use ofnitrogen is conducted during 4.5-6 minutes. Finally, a warm-air dryingcycle at 82° C. in the presence of N₂ during 8 min is performed.Thickness of a finished coating film layer varies depending on theconcentration of the amine agent in IPA. At 0.5%, it is <500; at 1.2% itbecomes about 600-800; and at 2% it comes up to about 900-1000.

The coating agent and the coating after depositing exhibit goodelectrical conductivity and isolate or shield the mask (reticle) fromelectric static field. The film is more than 95% permeable for I-lineand deep UV (DUV) light sources; it does not decay and is not damaged bythe illumination. Depositing the film eliminates the problem of apotential difference between the quartz substrate and the chrome patternand thus removes the cause of the ESD. The shielding effect of thecoating prevents the reticle from being induced by the ESC. The maskwith the coating can shield the mask from ESD as high as 8KV.

As shown in FIG. 2, the pellicle membrane 18 comprises a base(substrate) 24 and a layer 26. The base is made of nitro-cellulose(C₆H₉O₇N)_(n), and the layer 26, in accordance with the principles ofthe present invention, is an antistatic, dissipative, conductive film.The material for the layer 26 is a quaternary amine (R₄N)⁺Cl⁻, with amolecular weight of about 200˜500. A cationic polymer of the materialhas chemical formula:

The layer 26 can be deposited on the base by immersion of the base intoa 0.2-1.5%, preferably 0.5% (w/w) water or low-end alcohol solution ofthe agent. It is preferable, however, to deposit the agent solution bythe spin coating process discussed in the above. Since the cellulosesubstrate 24 is polarized, the antistatic film exhibits goodadhesiveness thereto. Preferable thickness of the finished coating isbetween 100 and 500. The initial and final phases of depositing areillustrated by FIGS. 3 a and 3 b. FIG. 3 a shows amine/DIW solution 30placed in the middle of a blank of the substrate 24. A cross-section inFIG. 3 b illustrates the pellicle 18 with the film 26 evenly spread overthe substrate 24.

The pellicle 18 is supported above the reticle 10 by a frame 28preferably made of aluminum. It is suggested that an antistaticconductive coating be deposited on the frame. A 1.5% (w/w) solution ofthe above-discussed amine agent in DIW is prepared, and the frame 28 iscoated by immersion into the solution placed in a container 32 (FIG. 4).

It can be seen in FIG. 2 a that the frame 28 is coated from both outside(a layer 34) and inside (a layer 36). The coating has thickness of about1.5μ. The pellicle 18 is attached to the frame 28 by glue designated as38.

As schematically shown in FIG. 5, the aluminum frame 28 takes on itselfelectrical charges from both reticle and pellicle and thus lowers downto a minimum the danger of ESD affecting the reticle/pellicle structure.

While the preferred embodiments of the present invention have beendisclosed herein above, it is to be understood that these embodimentsare given by example only and not in a limiting sense. Those skilled inthe art may make various modifications and additions to the preferredembodiments chosen to illustrate the invention without departing fromthe spirit and scope of the present contribution to the art.Accordingly, it is to be realized that the patent protection sought andto be afforded hereby shall be deemed to extend to the subject matterclaimed and all equivalence thereof fairly within the scope of theinvention.

1. A static resistant reticle for use in integrated circuit lithographicprocess comprising: a quartz substrate, a patterning layer placed on thesubstrate, and a film coating of an antistatic conductive materialcovering the substrate and the patterning layer, whereby a shieldingeffect protecting the reticle by eliminating potential difference on thereticle is attained.
 2. The static resistant reticle as claimed in claim1, wherein the patterning layer is made of chrome.
 3. The staticresistant reticle as claimed in claim 1, wherein the antistaticconductive material used for coating includes a quaternary amine(R₄N)⁺Cl⁻ with M.W. of about 300-500.
 4. The static resistant reticle asclaimed in claim 3, wherein the amine used for the coating has chemicalformula:


5. The static resistant reticle as claimed in claim 3, wherein the amineis used in the form of a 0.2˜5% solution (w/w) in isopropyl alcohol ordeionized water.
 6. The static resistant reticle as claimed in claim 5,wherein the solution is deposited on the reticle by a spin coatingprocess.
 7. The static resistant reticle as claimed in claim 5, whereinvapor coating is used for depositing antistatic material on the reticlesubstrate.
 8. The static resistant reticle as claimed in claim 7,wherein the reticle with the film deposited thereon is subjected todrying in a Marangoni-type dryer in the presence of nitrogen for theduration of 4.5-6 min.
 9. The static resistant reticle as claimed inclaim 8, wherein the Marangoni-type drying is followed by a warmair-drying at about 82° C. in the presence of nitrogen for about 8 min.10. The static resistant reticle as claimed in claim 1, wherein the filmcoating has thickness of about 500-1000.
 11. A pellicle used forpreventing dust particles from lying on a reticle and for defocusing thedust particles lying on the pellicle, the pellicle comprising: apellicle membrane; and a pellicle mounting system including a frame, onwhich the pellicle membrane is attached and with which the pellicleestablishes contact with the reticle, the pellicle being coated by anantistatic electro conductive coating to prevent ESD on the reticle fromoccurring.
 12. The pellicle as claimed in claim 11, wherein the membraneis made of nitro-cellulose.
 13. The pellicle as claimed in claim 11,wherein an antistatic material used for the coating is a quaternaryamine (R₄N)⁺Cl⁻ with M.W. of about 200-500.
 14. The pellicle as claimedin claim 13, wherein the quaternary amine has chemical formula:


15. The pellicle as claimed in claim 13, wherein the coating isdeposited by an immersion of the membrane into a 0.2-1.5% (w/w) water orlow-end alcohol solution of the amine.
 16. The pellicle as claimed inclaim 11, wherein the coating is deposited by spin coating on themembrane.
 17. The pellicle as claimed in claim 11, wherein the coatingdeposited on the film preferably has thickness of about 100 Å.
 18. Thepellicle as claimed in claim 11, wherein the frame is made of anodizedaluminum.
 19. The pellicle as claimed in claim 18, wherein the coatingis deposited on the frame by immerse coating of the frame withsubsequent natural drying.
 20. The pellicle as claimed in claim 19,wherein the coating deposited on the frame has thickness of about 1.5μm.
 21. A static resistant reticle-pellicle structure for use inintegrated circuit lithographic process comprising: a reticle with aquartz substrate and a patterning layer placed on the substrate, apellicle with a pellicle membrane, and a pellicle mounting systemincluding a frame, on which the pellicle membrane is attached and withwhich the pellicle establishes contact with the reticle, wherein anantistatic electro conductive coating is deposited on the reticle andpellicle to thereby shield the structure and prevent ESD on the reticlefrom occurring.
 22. The static resistant reticle-pellicle structure asclaimed in claim 21, wherein an antistatic material used for the coatingis a quaternary amine (R₄N)⁺Cl⁻ with M.W. of about 200-500.
 23. Thestatic resistant reticle-pellicle structure as claimed in claim 22,wherein the amine used for the coating has chemical formula: